Oct, 12 2008

Electrical

In this edition of Newsletters that Teach we're going to introduce you to some PLC basics and hopefully shed some light on a few dark areas.

Let's start with the good old days and how things used to be. If you look at the drawing below, the left hand side shows a circuit wired to turn a lamp on and off. What you see below is the classic "Start/Stop Station". The lamp will come ON when the START button is depressed, and will remain ON until the STOP button is depressed.

The drawing on the right is exactly the same as the drawing on the left, but it is drawn as a ladder. The ladder style of drawing became the standard many years ago because it is much easier to understand.

In the good old days, miles and miles of wires and numerous relays were needed to control all of the equipment in an industrial plant. You can imagine the mess of wires. A few years ago (okay, 35 years), engineers at Modicon invented a "machine" to eliminate all unnecessary relays and associated wiring. That "machine" was the PLC. (Allen Bradley PLC 5 is shown here.)

The PLC is much like your home computer, but has a few additional pieces.

The PLC has modules where input devices such as switches and output devices such as lamps are connected.A program written in a ladder format is stored in the PLC. This program provides instructions on what to do with the output devices based on what is happening with the input devices.

Let's not get into the program at this stage in time. All you need to know is that the program looks just like the old style ladder drawings of the "good old days".

We're going to take the switches and the lamp from our original "good old days" circuit and connect them to the PLC. Have a look below to see how the connections are made.

Seems simple enough? The ladder on the right is the written program stored in the PLC memory.

Let's put both circuits side by side so you can compare. Will they both operate identically?

You probably have already noticed that the relay "C" has been eliminated in the PLC configuration. It's not obvious from the drawing but lots of wire has been eliminated also.